1. Field of the Invention
The invention relates to a high voltage impulse wave generator and more particularly to such a generator for generating the 1.2.times.50 test impulse wave specified in DOD-STD-1399 Section 300, IEEE Standard 587-1980, and other standards, to be applied to electrical and electronic equipment supplied by ac or dc power, to determine its vulnerability to lightning and/or switching transients.
2. Background Art
Ungrounded shipboard electrical equipment is subjected to both line-to-line and line-to-ground impulse waves as specified in MIL-STD-1399 Section 300 to test its vulnerability to surge voltages. Existing commercially available surge generators have limitations that render them inadequate to perform these tests. The equipment described herein has the capability of performing surge voltage tests in accordance with U.S. Navy requirements.
Test surges are generated in presently available surge voltage generators by charging a high voltage capacitor to the desired voltage level and then discharging it through a triggered switch such as an ignitron or a spark gap. The test surge waves are applied to equipment operating on ac or dc power lines by series injection or by capacitive coupling.
In the series injection method, the capacitor is discharged into a resistor and air core inductor connected in parallel which are wired in series with the power line. The surge, which is developed across the parallel load, is superimposed on the line voltage with this circuit configuration. The surge energy divides between the input impedance of the equipment under test (EUT) and the power source impedance. The surge width is determined by the component values chosen for the discharge capacitor and the resistor-inductor combination. The impedance of the resistor and inductor is made very low to reduce the effect of the system impedance and the input impedance of the EUT on the surge width. A capacitor of 10 uF or greater is connected across the power line before the parallel load and the EUT. The capacitor presents a low source impedance to the surge and ensures that nearly all of the surge voltage appears across the input of the EUT; it also protects the power line and other equipment operating on it from the surge.
There are several difficulties in the series injection method. The series inductor carries line current continuously; hence, its wire gauge must be selected accordingly. In instances where the load draws hundreds of amperes, the inductor will be quite large and can also cause a phase voltage unbalance in three-phase power systems that could be substantial in 400 Hz applications. Inductors would have to be added in the other two phases to reestablish voltage balance. A high line voltage drop would also occur depending on the inductor value. Since the surge voltage is superimposed on the line voltage, it will exceed a preset peak voltage at the 240.degree. point in one phase when applied at the zero-crossing in another phase. Furthermore, line-to-ground surges cannot be generated with this method.
Voltage surges are also applied to power lines through coupling capacitors. This method is applicable where the power lines can operate normally with the coupling capacitor connected across them. However, as in the series injection method, the surge voltage is superimposed on the line voltage and will exceed a preset peak voltage at the 240.degree. point in one phase of a three-phase power system when applied at the zero-crossing in another phase. Also, methods do not now exist for performing line-to-ground surge voltage tests on equipment operating on ungrounded power systems such as are found aboard U.S. Navy ships. Moreover, reasonably priced, compact surge back filters to provide protection for the power source when conducting surge tests are not available.
These problems are addressed in part by the surge voltage generator described by the present inventor in U.S. Pat. No. 4,868,505, the disclosures of which are incorporated by reference herein. The generator of the '505 patent gives excellent results, particularly with 115-125 Vac, 60 Hz, single phase electrical equipment. However, there is a continuing need for additional and more flexible types of surge voltage generators which are applicable to a wider variety of electrical systems.